1. Field of the Invention
The present invention relates to a charged particle beam extraction system and method for extracting an ion beam, e.g., a proton or carbon ion beam, to be irradiated toward an irradiation target.
2. Description of the Related Art
There is known a therapy method for irradiating an ion beam, e.g., a proton or carbon ion beam, to an affected part in the body of a patient, such as a cancer. An ion beam extraction system for use in such therapy comprises an ion beam generator, a beam line, and an irradiation device. An ion beam accelerated by the ion beam generator reaches the irradiation device, which is installed in a rotating gantry, through a first beam line and a second beam line, the latter being also installed in the rotating gantry. The ion beam is extracted from the irradiation device and irradiated to the affected part in the patient body. Known examples of the ion beam generator include means for circulating the ion beam along an orbit, means for bringing betatron oscillation of the ion beam into a resonant state outside the separatrix of resonance, and a synchrotron (circular accelerator) provided with an extraction deflector for taking the ion beam out of the orbit.
The therapy using an ion beam, for example, the treatment with irradiation of a proton beam, is based on a characteristic that, at the time when protons are stopped, most of energy of the proton beam is released and the so-called Bragg peak is formed. By utilizing such a characteristic, input energy of the proton beam is selected to stop protons near the affected part in the patient body so that most of the energy (absorbed dose) is given only to cells of the affected part.
Usually, the affected part has a certain thickness in the direction of depth from the body surface of the patient (i.e., the direction of travel of the ion beam; hereinafter referred to simply as the “direction of depth”). In order to effectively irradiate the ion beam over the entire thickness of the affected part in the direction of depth, the energy of the ion beam must be controlled so as to form a comparatively wide and flat range of absorbed dose in the direction of depth (i.e., a spread-out Bragg peak; hereinafter abbreviated to an “SOBP”).
As one of means for forming the SOBP, there is a ridge filter having a plurality of thickness components depending on positions in a direction perpendicular to the direction of travel of the beam (see, e.g., p. 2078 and FIG. 31 of Non-Patent Document 1; “REVIEW OF SCIENTIFIC INSTRUMENTS”, Vol. 64, No. 8 (August 1993), pp. 2074-2088 and FIGS. 30, 31 and 45). When the ion beam passes through a thin portion of the ridge filter, the energy of the ion beam is slightly attenuated and the Bragg peak is produced in a deep position inside the body. When the ion beam passes through a thick portion of the ridge filter, the energy of the ion beam is largely attenuated and the Bragg peak is produced in a shallow position near the body surface. As a result of spatially changing the position in the direction of depth where the Bragg peak is produced in such a manner, a dose distribution in the direction of depth of the affected part becomes uniform and the SOBP is formed.
As another means for producing the SOBP, there is a range modulation wheel (hereinafter abbreviated to an “RMW”) which includes a plurality of blades arranged around a rotary shaft and each having a thickness changing step by step in the circumferential direction (see, e.g., p. 2077 and FIG. 30 of Non-Patent Document 1). The plurality of blades are mounted to the rotary shaft. The RMW has an opening formed to penetrate between the adjacent blades. The RMW is started, for example, to rotate from a state where the opening is set in a travel path of the ion beam (hereinafter referred to as a “beam path”). The rotation of the RMW causes the opening and the blade to alternately intersect the beam path. When the ion beam passes through the opening, the energy of the ion beam is not attenuated and the Bragg peak is produced in the deepest position inside the body. When the ion beam passes through the blade, the position of the Bragg peak is changed depending on the thickness of the blade. More specifically, when the ion beam passes through a thicker portion of the blade, the energy of the ion beam is attenuated to a larger extent and the Bragg peak is produced in a position of the affected part nearer to the body surface. With the rotation of the RMW, the position where the Bragg peak is formed varies cyclically. As a result, a flat SOBP can be obtained over a comparatively wide region in the direction of depth, looking at the beam energy integrated over time.
As still another means for forming the SOBP, there is an intensity modulation (current modulation) method. This method is to obtain a desired dose distribution in the direction of depth by controlling the amount of the extracted ion beam (hereinafter referred to as the “beam amount”) and the energy of the ion beam (hereinafter referred to as the “beam energy”). The control of the beam energy is performed, for example, by changing the setting of the synchrotron (or by changing the thickness of a beam energy absorber disposed in the irradiation device). The beam amount is controlled by counting the dose by a dose monitor installed in the irradiation device and making a shift to a next level of beam energy when the counted dose reaches a predetermined amount. Thus, a flat SOBP can be obtained in the direction of depth by controlling the beam amount and the beam energy so that a dose distribution is uniform in the affected part of the patient body in the direction of depth.
As still another means for forming the SOBP, there is a scanning method of irradiating an ion beam to the affected part in the patient body while scanning the ion beam with a scanning magnet (see, e.g., p. 2086 and FIG. 45 of Non-Patent Document 1). According to the scanning irradiation method, a dose distribution in the direction of depth is adjusted by changing beam energy of a thin beam, and a dose distribution in the planar direction is adjusted by changing the beam position in the planar direction by the scanning magnet. A dose distribution in the affected part of the patient body is made uniform with superimposition of all the beams. The scanning irradiation method can also produce a uniform SOBP in the affected part in the direction of depth.